Re-Os dating for mineral deposits

Working out when a mineral deposit formed is a key part of building a conceptual model. These conceptual models are the backbone of an exploration strategy.

Here's a quick primer on one technique: Re-Os dating of sulphides.

This one is very useful because you can actually put a number on a sulphide mineral itself. Not to say that it is always drama free. All techniques have their advantages and disadvantages.

Detail of molybdenite in quartz from Moly Hill Mine, Quebec, 15mm wide. Image courtesy of J.Chapman, Wikimedia Commons

Rehmium-Osmuim geochronology

Rhemium decays to osmium. There's a beta decay involved, which means a neutron releases an electron and antineutrino and converts to a proton. By losing a proton, the nucleus is now atomic number 76 (one more than Re's 75) and so becomes an isotope of osmium, Os187.

Rhemium is a great element to date ore deposits with because it is more or less the only radioactive isotope that partitions into sulphide minerals. This stands the Re-Os technique in contrast with other methods like U-Pb, as U generally partitions into silicate minerals like zircon more easily than sulphides.

Rhemium has a half life of over 42 billion years, meaning it's good for geological processes, as long as you have very sensitive mass spectrometers that can measure infinitesimally small amounts of isotopes. Amazing stuff.

Molybdenite is often the mineral that is most used for geochronology purposes as it has high concentrations of Re and Os. One study summarised it as:

...molybdenite (MoS2) ... constitutes a particular case within sulphide minerals because it contains high Re (in the ppm range) and 187Os (at ppb levels), but almost no initial or common 187Os... . These unique characteristics explain why Re-Os molybdenite dating using the whole mineral approach is currently the most widely used single mineral geochronometer in ore deposits.

Some examples

Simplified geological map of Ireland including the location of the Navan and Lisheen deposits. From Yesares et Al 2019

Irish Pb-Zn deposits have seen some good metallogenic studies in which Re-Os dating has been undertaken. Lisheen is the second-largest base-metal deposit in Ireland. Re-Os dating of pyrite from the deposit showed that ore-stage pyrite at 346.6 ± 3.0 Ma, shortly after the host rock was deposited, as is typical for these Irish-type carbonate replacement type deposits. A key advantage of knowing that mineralisation occurred soon after deposition is that the Re-Os ages overlap with the ages of lower Carboniferous volcanic rocks nearby, suggesting magmatism could be a driving force for the hydrothermal activity.

Another example is from the Tianshan, western China, where strataform sulphide orebodies within mafic volcanoes have been dated using Re-Os. In this study of the South Meiling Cu–Zn deposit, a Re-Os isochron age of 434.2 ± 3.9 Ma was obtained from chalcopyrite, suggesting the volcanic massive sulphide (VMS) deposit was related to basin formation above a Tethyan subduction system.

Molybdenite dating has also been applied at the Vulcan prospect in the Olympic Cu-Au Province, South Australia. This study, which I was part of, gave an age of 1586 ± 8 Ma from molybdenite within hematite-rich brecciated granite samplesthe mineralisation. This showed that mineralisation in this prospect was related to the major Mesoproterozoic magmatic and hydrothermal event that included Olympic Dam. This result gave renewed focus to further exploration of this prospect.

187Re vs 187Os isochron plot for samples from the Vulcan Prospect. See also Reid and Jagodzinski 2012

Geochronology tips

The best geochronology is done when the samples are well characterised beforehand. By this I mean the thin section is described and photographed and the mineral textures understood. In the case of Re-Os the more Re in a mineral the better quality the analyses will be and the more precision on any age calculation. This is especially important for minerals like pyrite or chalcopyrite. These can sometimes have very low abundance of Re making them impossible to date. Best bet is to get in touch with one of the reputable laboratories around the place, like Durham University and others. These days there is also research into using the laser ablation system for Re-Os to provide in situ geochronology, which could be excellent if the uncertainties can be minimised. So if you have sulphides and a need to know the age, have a think about the Re-Os technique.